The present invention relates to an improved friction gear and more particularly, to an improved friction gear which has an infinitely variable transmission ratio and which is utilized to transmit rotational movement from a driving shaft to a driven shaft.
A friction gear for transmitting rotational movement from a driving shaft to a driven shaft, with an infinitely variable transmission ratio within certain limits, is disclosed in Austrian Pat. No. 242,457. The rotational axes of the shafts in this friction gear are disposed parallel to one another. One shaft is provided with an axial bearing surface while the other shaft has rollers that are spaced from the rotational axis of the other shaft and that bear on the bearing surface of the one shaft. The rollers are driven rotatably around their axes which extend parallel to the axis of the other shaft and which all lie on a circle concentric with the axis of the other shaft. The means for mounting the rollers are driven, through toothed wheel gearings, at an angular speed which is equal to one half the angular speed of the other shaft, with their direction of rotation being opposite to direction of rotation of the other shaft. An adjustment of the transmission ratio is affected by parallel displacement of the driven and driving shafts relatively to one another. This adjustment of the transmission ratio, by parallel displacement of the shafts, is a disadvantage with this known friction gear. The need to be able to make such a parallel displacement of the shafts means that driving and driven shafts cannot be directly interconnected, but rather special means, such as additional intermediate gears, must be provided in order to make a fixed coupling between the driving and driven shafts. An additional disadvantage is that the transmission ratio is only variable within certain limits, and in fact, extends only from slow running (although not 0) through the ratio of 1:1, into fast running. A use of this known friction gear in the range of 1:0 is not possible, and in addition, reversal of the direction of rotation is also impossible. Moreover, when the transmission ratio is 1:1, the rollers are rotated in situ. As a result, irregular wear on the bearing surface is produced. This leads to an irregular running when transmission ratios other than 1:1 are set. Still another disadvantage of this known friction gear is that an adjustment of the transmission ratio is impossible when the shafts are not rotating because this would require movement of the rollers transversely of their running direction. This disadvantage also applies, in part, with transmission ratios which are close to 1:1.
Another prior friction gear is disclosed in U.S. Pat. No. 3,033,048. This friction gear includes driving and driven shafts whose axes are parallel but offset from one another. Axial bearing surfaces are provided in the facing ends of the shafts and are coupled to one another through a roller. This roller is mounted on a line which perpendicularly intersects the axes of the driving and driven shafts, and the rotational axis of the roller extends obliquely of this line, in the direction of the line, so that the roller always acts with its edges on the axial bearing surfaces. The driving and driven shafts are arranged fixed relatively to one another, and it is thereby necessary to accept the disadvantages which usually occur with such arrangements. In this regard, only one roller can be arranged between the axial surfaces so that the torques which can be transmitted between the shafts are limited. This disadvantage could, of course, be partly overcome by an increased expenditure, i.e., by the use of several such gears connected in parallel, but oftentimes this may not be a viable alternative.
An additional disadvantage is that, with any particular transmission ratio, the running track of the roller, or its edge, on the axial bearing surface is always the same so that local wear and groove formation occurs. Furthermore adjustment of the transmission ratio when the shafts are stationary is impossible because this would mean sliding the roller transversely of its running direction. This disadvantage is also partly applicable at slow running speeds between the roller and its bearing surface.
In the present invention, an improved friction gear, with an infinitely variable transmission ratio, is provided which overcomes the disadvantages of these prior friction gears. More specifically, the improved friction gear of the present invention comprises driving and driven shafts which are nonadjustably fixed relative to each other, which are interconnected and which have their rotational axes parallel to and spaced from one another. One of the shafts is provided with a substantially axial bearing surface on its facing end. The other shaft is provided with at least two rollers that are spaced from the rotational axis of the other shaft and that bear on the bearing surface of the one shaft. The means for mounting the rollers holds and aligns each of the rollers such that its rotational axis is always directed toward a straight line which lies in a plane determined by the two rotational axes of the shafts.
The means for mounting the rollers includes arms which couple the rollers, mounted so as to permit pivotable movement, with an axially symmetrical guiding surface. The longitudinal axis of this guiding surface is a straight line which is parallel to the axes of the shafts and which lies in the plane defined by the two rotational axes of the shafts. To vary the transmission ratio, the guiding surface is adjustable, in a plane that is defined by the two rotational axes of the shafts and that is perpendicular to the guiding surface. However, regardless of the position of guiding surface, the roller axes always intersect the longitudinal axis of the axially symmetrical guiding surface.
The axially symmetrical guiding surface, which causes the rollers to run on cycloidal tracks on the bearing surface of the one shaft, can be constructed in various ways. For example, it may form an axial, concentric groove which positively guides the roller axes, through the arms. The axially symmetrical guiding surface can also be the external cylindrical surface of a ring and the arms may or may not be held in a bearing position, against the guiding surface by springs.
As described above, several rollers may be simultaneously used with the bearing surface in the improved friction gear of the present invention so that a comparatively high torque may be transmitted. This is a particular advantage because it is thus possible to operate the improved friction gear in the vicinity of transmission ratios of 1:0 whereat slow rotational speeds occur, and whereat, in practice, relatively large torques are generally to be taken up. Thus a friction gear of the present invention can consequently be made comparatively or relatively small.
An additional advantage consists in that each roller, during each rotation, travels over a different track on the bearing surface. In other words, as the two shafts rotate relatively to one another, the tracks of the rollers do not follow the same circular path on the bearing surface whereby uniform degree of wear on the bearing surface is assured. Therefore even when the friction gear of the present invention is run a long time at one particular transmission ratio, no grooves will be formed on the bearing surface.
Another significant advantage of the improved friction gear of the present invention is the fixed, parallel arrangement of driving and driven shafts. This permits the fixed installation, without intermediate gears, of the shafts, and this can be accomplished because the means for adjusting the transmission ratio does not serve for transmitting rotational movement between the shafts. As noted above, this adjusting means comprises the axially symmetrical guiding surface which guides the arms serving for the alignment of the rollers and thus the roller axes. Moreover, since the axially symmetrical guiding surface is adjustable, as a practical matter, independently of the means for transmitting the rotational movements between the shafts, an adjustment through 0 is possible, not only theoretically, but also practically. Thus, it is possible that the friction gear of the present invention can reverse the direction of the rotational movement of the driven shaft so that the improved friction gear can also be used as a reversing gear.
Another advantageous feature of the improved friction gear of the present invention is the adjustability of the transmission ratio when the driving and driven shafts are stationary. During such an adjustment, each of the rollers merely swivel about its axis which is disposed at right-angles to the bearing surface. Such a swivelling movement is possible because of the small extent of the contact surface between the rollers and the bearing surface. Furthermore the coupling of the rollers with the axially symmetrical guide surface is yieldable so there is only minimal pre-adjustment and a swivelling of the rollers unto their track, as determined by the new transmission setting, when the driving shaft is again set in motion. In no circumstances, however, with the friction gear of the present invention, is there any displacement of the rollers transversely of their running direction during an adjustment of the transmission ratio.
Still another important advantageous feature of the improved friction gear of the present invention results from the fact that power transmission no longer takes place in the running direction of the rollers, as is the case with the friction gear described in the previously mentioned U.S. Pat. No. 3,033,048, but also takes place, quite necessarily, in a direction transverse to the running direction. This has the effect, especially when the bearing surface is soft, that a strong, positive pick-up occurs, without any clamping effect, as a result of the rollers being pressed into the bearing surface.
The guiding forces for the rollers are extremely small since the driving forces take effect radially on the pins which are rotated for guiding the rollers. The pins or journals which are used in some of the embodiments of the present invention to mount the rollers are disposed in a cylindrical element which, in turn, is mounted on the other shaft so as to be secure against rotation but displaceable axially. Furthermore, with the improved friction gear of the present invention, the ratio between running forces and rotational forces of a roller on the bearing surface is favorable, even with the most extreme transmission ratios, so that there is no problem with any abrasion, premature wear or even a binding of the gear. Thus the friction gear according to the present invention runs extremely easily and with minimal wear regardless of the transmission ratio that is set. In this regard, the pressure at which the rollers are applied to the bearing surface can be very easily controlled since the biasing of the rollers to the bearing surface is produced by means of a spring.
In another embodiment of the present invention, one of the shafts comprises two substantially axial bearing surfaces facing one another. A plurality of bearings, each including a pair of rollers, are disposed between the facing bearing surfaces and are arranged so that one roller runs on the one bearing surface and the other on the other facing bearing surface and so that the pressure-applying forces of the rollers of one pair are cancelled out. The advantage of this embodiment is that there may be a doubling of the torque that can be transmitted with only a relatively small increase in the overall size of the friction gear.
In still another embodiment of the present invention, the axes of the rollers are disposed at an inclination to the longitudinal axis of the axially symmetrical guiding surface and to relatively the bearing surface on the shaft. The rollers are bevelled and the bearing surfaces of the rollers are preferably conical. The bearing surfaces of the rollers and the bearing surface on the shaft thus largely form a bevel gear so that frictional effects due to rotation of these bearing surfaces, relatively to one another, are minimal.
Accordingly it is a primary objective of the present invention to provide an improved friction gear of the type described, wherein the rollers, with a particular transmission ratio, do not always run on the same tracks on the bearing surface, wherein the driving and driven shafts are arranged fixed relatively to one another, wherein the improved friction gear is capable of being used as a reversing gear and wherein, an adjustment in the transmission ratio can take place when the shafts are not rotating.
These and other objects and advantages of the present invention will become apparent from the following description of the preferred embodiments of the present invention described in connection with the accompanying drawings.